1. Field of the Invention:
This invention relates, in general, to electrical connectors and, more specifically, to load-break bushings for use with plug-in type electrical connectors.
2. Description of the Prior Art:
The recent interest in underground distribution power systems for residential areas has resulted in the development of plug-in electrical connectors which enable the high voltage shielded cables of the electrical distribution system to be quickly connected and disconnected from electrical apparatus, such as distribution transformers and electrical switches. Such connectors are, typically, characterized by an elongated male contact pin which is electrically connected to the high voltage cable of the distribution system. A housing surrounds the male contact pin and has an elbow configuration with a "D" hook located at the apex of the elbow. The "D" hook is located in alignment with the longitudinal axis of the male contact in the connector, enabling the operator to grasp the "D" hook with a hot stick tool and forcibly urge the male contact pin of the electrical connector into or out of electrical engagement with a corresponding female contact located, for example, in a transformer mounted bushing.
In the past, load-break bushings have been susceptible to failure since, under certain conditions of loading and with certain transformer connections, the full three phase voltage may appear across the bushing contacts causing damage thereto. For this reason, the electrical industry is in the process of adopting a new standard requiring a so-called "three phase-rated" bushing.
In actual operation, an arc is struck between the male and female contacts during load-make and load-break operations; with the duration of the arc determined by the closure or separation speed utilized by the operator. The relatively slow speed utilized by the operator normally presents no problems since the contacts can be designed to withstand the arc that is drawn between them as they are moved toward engagement or drawn apart. However, if a fault, such as a short circuit, exists on the distribution system, the closure speed becomes increasingly important since the arc energy level increases proportionally to the magnitude of the abnormally high fault current. In addition, the high voltages utilized in such distribution systems result in the arc being drawn over a longer distance. The subsequent longer arcing time and increased arc energy during fault close-in frequently damages the conducting elements of the bushings and presents a substantial danger to the operator. The arc that is drawn during a fault close-in causes a substantial quantity of gas to be generated by the arc quenching material commonly used in these types of connectors thereby creating extremely high pressures within the bushing housing which impedes the closure of the contacts and frequently results in a blow-back of the contacts towards the operator.
It has been proposed to reduce the duration of the arc by closing the contacts at a high rate of speed. A typical method of closing the contacts at a high rate of speed, as shown in U.S. Pat. Nos. 3,542,986; 3,930,709 and 3,958,855, utilizes the arc produced gases to operate a piston in such a manner so as to accelerate contact closure. Another commonly used method, shown in U.S. Pat. Nos. 3,945,699 and 3,957,332, utilizes a magnetic solenoid principle wherein a magnetic means, such as a coil, surrounds a movable contact within the bushing and, in response to the arc current flowing therein, drives the contacts together at a high rate of speed. In addition, U.S. Pat. No. 3,989,341 uses a spring means to move a tubular conductor from an interim position towards a cooperating contact pin to effect a rapid closing operation. The effect of the high closing speed in these types of devices is to reduce the arcing time. The same effect can be achieved with slower moving contacts if the arc length can be reduced.
Thus, if would be desirable to provide a load-break bushing which minimizes arc generation during load-make and load-break operations. Furthermore, it would be desirable to provide a load-break bushing which provides effortless closure of the contacts in instances where a fault condition exists on the distribution system.